1. Field of the Invention
The present invention relates to a method of forming a dielectric film for a semiconductor device and, more particularly, it relates to a method of forming a laminated film which can be used as a dielectric film of a capacitor and which includes oxidized tantalum layers.
2. Description of the Related Art
Various new processes have been developed these days to make semiconductor devices higher in performance and more highly integrated. The capacitor forming process is one of them and it is deemed as being important.
The capacitance C of the parallel-plate capacitor can be expressed as follows: EQU C=e.sub.0 .multidot.e.sub.r .multidot.S/d
wherein e.sub.0 represents the vacuum dielectric constant, e.sub.r the relative dielectric constant of the dielectric substance, S the area of the plate and d the film thickness of the dielectric substance.
In order to increase the capacitance of the capacitor, therefore, the film thickness of the dielectric substance may be reduced. In the case of the small capacitance semiconductor device such as 1 M DRAM now available, therefore, its capacitor capacitance which becomes smaller as the capacitor is made finer can be kept not reduced when its parallel-plate capacitor is made thin or the film thickness of its dielectric substance is decreased.
In the case of 4 or more Ms DRAMs, however, it has become impossible to make the memory cell thin, from a physical viewpoint, by using a laminated film of SiO.sub.2 and Si.sub.3 N.sub.4, which is now used as dielectric film, as it is. The reasons are as follows:
1) SiO.sub.2 has a low relative dielectric constant of 3.8, and even if the laminated film is made thin, the capacitance thereof cannot be made large enough.
2) When the laminated film is made thinner than 50 .ANG., leak current is increased.
In order to increase the capacitance, therefore, it has been tried to increase the area (S) of the plate or surface area of the capacitor. In other words, the capacitor has been tried to have a three-dimensional structure. Early days when it was wanted that the surface area of the capacitor was made as large as possible, the capacitor of the trench type was studied. In the case of the capacitor of this type, however, its signal to noise ratio (or S/N ratio) is lowered by a film formed of oxidized silicon. Therefore, the capacitor having a structure of the stacked type has been used these days. In addition to forming the stacked capacitor, this stacked type structure has been applied to the trenched Si to form a capacitor of stacked and trenched structure.
As 16- and 64-M DRAMs are developed, the devices are made finer and their capacitors are made thinner. In the case of the 64-M DRAM, for example, it is believed that its capacitor size becomes 1.5 mm.sup.2 and that its film thickness becomes smaller than 50 .ANG. when calculated in terms of a oxide film thickness. In addition, it is asked that power consumed must be made as smaller as possible. Since signal charge capacity equals to a product obtained by multiplying electrostatic capacitance by operating voltage, the lowering of power source voltage must be corrected by increasing the electrostatic capacitance. In order to increase the surface area of the capacitor, therefore, capacitors of the fin, crown and chimney types have been proposed these days. It is however quite difficult to make these capacitors because their structures are complicated.
As means for solving this problem, attention has been again paid to those materials, which have a high dielectric constant, such as tantalum pentoxide (Ta.sub.2 O.sub.5), and studies have been conducted to make them practical (see a monthly "Semiconductor World", March issue, 1987 and May issue, 1990: and "J. Electrochem., Soc.", Vol. 136, No. 3, March, 1989).
Ta.sub.2 O.sub.5 is one of materials most worthy of attention because its relative dielectric constant is as high as 28. However, it is still impossible to form a film of fully satisfactory and high quality with this Ta.sub.2 O.sub.5. Even if dielectric film is made using Ta.sub.2 O.sub.5, therefore, its withstand voltage is still low. This is a problem to be conquered upon making Ta.sub.2 O.sub.5 practical.
The reasons why a high quality film cannot be made using Ta.sub.2 O.sub.5 are as follows:
1) Ta.sub.2 O.sub.5 is likely to be reduced by Si which serves as base material. This makes oxygen atoms short, thereby causing the withstand voltage of the film thus formed to become low. When a little more added from the viewpoint of standard free energy of formation, SiO.sub.2 =-0.8244 J/mol and Ta.sub.2 O.sub.5 =-1.191 J/mol. O in Ta.sub.2 O.sub.5 is therefore likely to be reduced by Si. Further, when CVD and sputtering manners are used, in which an organic Ta gas is mixed with an oxidizing gas for continuous deposition, a film thus formed is often short of oxygen, and the composition thereof deviates from the stoichiometric composition, because components reacted in vapor-phase are deposited. This leads to the lowering of dielectric constant.
2) When contaminants such as carbon are present in the film, they cause leak current, which lowers the withstand voltage of the film. Particularly a gas source of the organic Ta gas is decomposed in the plasma CVD process, for example, and carbon as an intermediate by-product is likely to be taken into the film and to remain there.
3) Even when deposition is to be made in grooves of high aspect ratio to increase capacitance, it cannot be made conformable (or uniform in holes and on inner walls severely stepped).
Ta.sub.2 O.sub.5 is quite high in physical property for a strong dielectric memory as described above, the barrier by which it is prevented from becoming practical cannot be broken yet.